A capacitive physical quantity sensor detects physical quantity such as acceleration, angular velocity, pressure and the like. Japanese Patent Application Publication No. 2002-40047, corresponding to U.S. Pat. No. 6,483,322, discloses a capacitive physical quantity sensor that has a self-diagnostic function for diagnosing output accuracy of the sensor.
A sensor like the above-described sensor includes a C-V conversion circuit having a differential amplifier circuit. One of a reference electric potential and a self-diagnosis electric potential is input into an input terminal of the differential amplifier circuit via a switch. The reference electric potential is used for detecting a capacitance change. The self-diagnosis electric potential is used for a self-diagnosis operation. The reference electric potential is different from the self-diagnosis electric potential. The above-described input terminal of the differential amplifier circuit is a non-inverting input terminal of the differential amplifier circuit for instance. When the self-diagnosis operation is performed, the switch is driven, and the self-diagnosis electric potential is applied. When a normal operation other than the self-diagnosis operation is performed, the reference electric potential is input.
When a plurality of sensors has an almost identical configuration, and when each sensor has a self-diagnosis function like the above-described self-diagnosis function, an output electric potential from each sensor may be approximately the same during a period of the self-diagnosis operation. In the above-described case, even if one of the sensors does short, the output electric potential does not change, and thus, it is difficult to determine whether the output is an appropriate self-diagnosis output or an abnormal output resulting from the abnormal short.